It stays still until someone removes them. If it's a cab it's more than likely to simply call for help.

And while it is still, it is now an obstacle for other empty self driving cars. Soon the troll has blocked an entire intersection or freeway using a combination of empty boxes and other trapped self-driving cars.

You underestimate the effort a good troll would be willing to put into this.

There are lots of simple things that one could do to a roadway to impede or endanger traffic. Consider, for example, the danger of leaving something sharp or slick on a tight turn where people often speed.

I've thought about this for a few years, and as near as I can tell it's either much harder than it seems, or there are fewer vandals than I'd have expected.